Decreasing deionization of gas tubes



Patented Nov. ll, 1952..

DECREASING DEIONIZATION F GAS TUBES Louis Malter and Edward 0. Johnson, Princeton,

N. J., assignors to Radio Corporation of America, a corporation of Delaware Application April24, 1951, Serial No. 222,694

18 Claims. (Cl. 315-337) This invention relates to methods o f the means for decreasing the de-ionization time of gas tubes and particularly of gas triodes.

As is known, the grid of a gas triode has conV trol for starting ionization of its gaseous lling. For example, if a positive pulse is applied to the negatively biased grid, the tube can be made to` lire at a particular instant when its diminishing bias passes from one side tothe other of. `the so-called control characteristic curve. However, once the tube is fired, the grid has substantially no controly for interrupting the discharge. even if its externally applied bias is made very negative with respect to. that curve. Therefore, the grid of an ionized gas triode is said to have lost control. t Y t The reason for loss of control is that once thefgaseous filling is ionized, the negatively biased grid will attract positive ions from the plasma until it.; is surrounded with enough of them to completely neutralize its bias. Thereafter an increase in lthe applied bias will merely attract enough. additional ions to once more neutralize it. Hencethe grid ceases to be effectual as a control electrode. The customary way of restoring the grid to control. as a starting, electrode is` (1) to stop the process ofv continuous ionization, i. e., to interrupt the discharge, by dropping the anode-cathode potential below the Avalue at whichY a going arc can be sustained, and (.2) to permit deionization (plasrna decay) to take place for a long enough. period, by keeping the potential below that value, to allowmmost of. the lons; which constituteV the grid sheathl tonrecombine with'free electrons and diffuse away from the grid. Thus the rate of deionization ofwits gaseousnlling determines the recovery time. ofh a'thyratron and thereby limits the repetition rate atvwhich it can. be cyclically operated as a switch, e. g., in a counter or sweep circuit. This limitation can be. clearly understood by considering the behavior of aconventional thyratron. in a relaxation-oscillator circuit, e. g., the. type of circuit shown in Figs. 5 and 6, whichmight be used to generate a sweep voltage for an oscilloscope. Assuming the presenceY of a conventional thyratron as the discharge tube of such a circuit, e. g., connected like the tubesY l0 and I0 in Figs. 5 and 6, and assuming that the otherwise readied circuit is rst placed in operation by 'closing la switch (see. S in Figs. 5 and 6). in ther charging circuit of the storage condenser (see C in Ithese figures),` the voltage across this condenser will rise exponentially for apredeterminedl time to a. value near to that required to re the gas tube at which time a 'synchronizing voltage will be applied to the control grid to lire the tube. Due to .the very low internal resistance of an ionized gas tube the condenser will discharge very rapidly. When it has discharged to below the extinguishing potential ofr the gaseous lling of the tube the plasma will start to decay. However, at the same time that it is decaying the plasma will continue to support conduction and therefore the voltage across the condenser will` drop Well below the extinguishing potential of the tube and to a value near zero volts. At this point, it is desirable. for a fresh cycle to start immediately but if. the plasma has not suiii'ciently decayed, the tube will continue to havefa very low impedance, even though. it does not fire again, and this will delay the start of re. charging. This delay will come to an end when the plasma density has diminished. to such a low value that the applied. negative bias of the grid is not suiciently neutralized to preventl it from cutting olf the space discharge of the tube for its then small anode potential. Thereafter the effective, i. e., the unneutralized, grid bias and the voltage across the condenser will both progressively increase. However, if the former is too slow with respect to the latter the anodecathode potential may reach. a value to `prematurely nre the tube. Where this condition exists, the only solution is to reduce the charging rate. This can be done only at the. cost of reducing either the repetition rate-of the oscillator, or the peak-to-peak amplitude of the generated saw-tooth voltage, or both. Thus it is seen that the deionizationtime can limit the repetition rate of a sweep circuit, and/or the rate of change ofv the sweeping voltage which it generates, and that it can do so both by delaying the start of each charging cycle, and secondly by restricting the rate at which recharging can take place. For this reason, it is very desirable to increase the speed with which deionization takes place in a gas tube.

The advantages which will be obtained by attaining a speeding up of. ionization will not be limited to sweep circuits but will extend to a variety of other circuits to which gas tubes are employed, e. g., counters,` Welder timers, transmitter-tube pulse-modulators.

Accordingly, it is an object of the present invention to decrease the de-ionization time of. a gas tube.

It is a further object of this invention to decrease the recovery time of the grid of a gas triode.

kIn general, this object is obtained according to the present invention through methods of and means for accelerating the plasma decay and therefore the dissipation of the grid ion sheath, by maintaining high electron temperatures within the tube during de-ioriization.

In the drawing, Figure 1 is a side view of a gas triode according to the present invention with its envelope partly cut away;

Figure 2 is a transverse sectional view of the triode of Figure l, the section being taken along the line 2-2 of Figure 1;

Figure 3 is a transverse sectional view of a modification of the triode shown in Figures l and 2, the section being taken as in Figure 2;

Figure 4 is a plot of two curves of plasma-density vs. time to show the difference in behavior between gas -triodes according to the present invention and conventional gas triodes; and

Figures 5 and 6 are schematic circuit diagrams employing gas triodes as in Figures l and 2 and 3 respectively.

The thyratron I9 shown in Figures l and 2 comprises a vacuum envelope I I containing a gas filling represented by the symbol G. Arranged co-axially within the envelope II there are in the order named: an indirectly heated cathode I2, a slotted cylindrical control grid I3 having a slot I 6, and an anode I4. Since the present invention does not relate to the use of any particular structural features and does not require any particular modificationor adaptation of structural features known to the prior art, the showing of tube Il] herein does not include many of the structural details which would be found in an actual physical embodiment, i. e., a heater for the cathode I2, support elements for the various electrodes, their respective leads and terminal pins, a Itubulation, etc. However, it is understood that these various necessary parts are to be incorporated into any actual tube embodying this invention and that they may be incorporated in accordance with the teachings of prior art. The portions of the tube Hl comprising its elements lI-I4 as taken alone constitute a conventional thyratron. It may be modified in one way according to the present invention by the addition of a tungsten filament I5 in the region between the control grid I3 and the anode I4 either alone or, as shown in Figure 2, in combination with a switch and. the complete recovery of grid control,

the temperature of the free electrons in the plasma rapidly fell from elevated temperatures of about 10,000 K. encountered in the going arc to temperatures in two different ranges: (1) one near the temperature of cathode I2, for electrons inside of the control grid I3, and (2) another near room temperature for the electrons outside of the control grid I3, i. e., between it and the anode I4. Investigation of this phenomenon led to the conclusion that early in the process of plasma decay the grids sheath of positive ions progressively increases in thickness, to make up for its diminishing density, and does so `to such an extent that the portions of the sheath which' surround the edges of the slot I6 come together so as effectively to divide the plasma into a first portion within the control grid and a second portion outside thereof. Due to the continued emission of hot electrons from the cathode I2 the free electrons of the first portion of the plasvdi) ma do not cool down below the cathode temperature. This maintains the electron diffusion rate at a high value and, since ions will diffuse at the same rate as electrons, the behaviour known as ambipolar diffusion, the diffusion rate of the relatively massive positive ions in said first portion of the plasma will also be maintained at a high value. yAs is known this will result in rapid plasma decay. The region outside of the grid, however, is isolated from the cathodes hot electrons. Therefore, the free electrons of the second portion of the plasma quickly cool to room temperature after which ambipolar diffusion and plasma decay proceed relatively slowly.

According to the present invention a means is provided for maintaining high electron temperatures in the portion of the plasma located between the grid and the anode to thereby accelerate the decay thereof. Attained reductions in the recovery time of the grid have been by a factor of the order of 2 3.

The electronheating means shown in Figure 2 comprises an incandescent filament I5 which may be used either alone or in combination with a subsidiary anode 2I, or some other electrode for performing its intended (as described below) function. When the filament I5 is used alone it should be connected to a source of heater current but otherwise may be left floating, for example, its end may be connected across -an ungrounded battery (see 30 in Figure 5) or to the terminals of the ungrounded secondary of the filament transformer. When so operated the filament I5 may to some extent perform its function of heating free electrons whether it is simply a heater, i. e., a hot wire, or is a copious emitter such as a thoriated tungsten cathode. However, it is far less efcient to use it as a mere heat source rather than -as a source of hot electrons. Used as an emitter the thermal velocities of its electrons can easily maintain the average tempera- `ture of the plasma electrons at some 2200 K. above room temperature without any substantial measurable increase in the average temperature of the actual structural parts of the tube.

While the filament I5 is shown in a preferred position in alignment with the slot I6, it is not necessary that it be so located. This is possible because of the fact that a continuous plasma tends to have the same average diffusion rate throughout its entire volume and that as a result the electron temperature of the region between the grid I3 and the anode I4 will be substantially uniform regardless of Where the filament I5 is placed. From the foregoing it is apparent that while a source of hot electrons is to be preferred over a simple heater as a means for maintaining the free electrons of the plasma at high temperatures nevertheless the latter is within the scope of this invention. Accordingly, it is possible. though not preferred, to use a resistive heater outside the anode, for example, wound about it or even outside of the envelope in which case the entire ltube II) would be heated. 4 It has been found that an indirectly heated oxide type of cathode may be used as a source of hot electrons instead of the filament I5. However, such cathodes have lower operating temperatures in a region near one thousand degrees K. Therefore a tungsten filament is to be preferred. rIfhis illustrates a principle of the invention, that a highA average thermal velocity for the hot electrons provided by the heater means is more important than a large number thereof.

Since the electron temperature in a going arc aprirono Y 5S. maybe off the order of'. m'tliousand; degrees K., iti' isv apparent that an. auxiliary; discharge canprovide electronsfof evenhigher temperaturethan an incandescent` tungsten. filament; This: can` be done in a number' of." ways. suchas by using'. the subsidiaryy anode 2|?v cooperation witlrthef4 lament |,5l or by an embodiment as in', Figure-` 2; Where an auxiliary? discharge isset upf-betweenl the filament |51andthesubsidiary'anode2|` to serve as*` a. source of." hot electrons its current' slfiould'v be limited to a` very small valuel sol thatit will not serve asl av copious source ot plasma. Obviously ifrthecurrent isz` not: so limited the grid will never recover` since its sheath will be:l con tinuously replenished by ions from the auxiliary decay of plasma in a conventionalgas tube over a period of time v starting at'theinstantwhen the anode-cathode potential'. isremoved.' For thisv curve'4 it` is noted that: (11i.` the: rate. of decayV is relatively slow; and (2.5i that the plasma density iinally-reacheszero. CurvezB represents; the decayzin a tube in which a: low-current auxiliary discharge` isw maintained*` during the extinguishwhen the. shield. Isis left:A floating, it; will con lect'A enough charged. particles, duerv to their 'there malvelocities, to'` become charged:` to apoint where the velocities: of like particles will be opposedxby its field and thus it too` will. automaticallyn assume which willk assist the hot electrons getting `-through the fora-minous portion I'I. Likewise the screen I9 may be polarized below the-potential? of the anode I4 'to cause, or to assist inY causing, a drift of hot electrons through the portion IIr` to- .Y ,'wardthe interiorof the anode I4?.

ment of the main discharge. For this curvev n noted that: (1li therate-offdecay'is-greatly in@ v creased; and (2) thatthouglr the plasma density quicklyy reaches a lowf value it never goes to zero. The current of the auxiliary discharge should' be sollowthattheabove-mentioned' love value' of the residual plasma-is solowf that the-aridi` can recover control despiteiits presence.V c Mostof! the elements 'nI Figure 35 areof: simi-lar construction to`l corresponding elementsln Figure' 2 andi therefore, bear the samereferenc'e-f. nu.- merals. However, the anode. I6." comprisesl af'oi'aminous` portion |71 which', as will be explained; servesV as an 'electron transparent region through which. hotl electrons may be admitted. into: the interelectrode space between the anode andz thev grid.l Outside ofthe anode4 Ill" and adjacent its foraminous portion Hr there1 is a box-shaped shield I9' containingA a subsidiary cathode W and ar subsidiary anodeZ'Il. 1

In the operation or the `sintesimenu; crxn'igure.

3l alsourceI of." potential suicient to sustain ago v 1.5.5 50

l cathode whichy has'no grid recovery timeproblem, sincevit has no grid, a considerable improvement ing: arobetweem thesubsidiary'cathode and anode il),A 2t" is connected. therebetween. Theconnection should be overl a limiting resister;y oi'f largevaluesince: a small discharge current willHsuf'- tlceior' the purposesy herein. In:V fact' this will causethe auxiliary dischar'gef-to betterL perform its intended function herein sinceits1smallmag nitude current will result in a somewhat higher arcdro'p and this will result in higher electron temperatures'. .As shown in- Figure 6i, the entire circuit comprising the subsidiary? ycathode and anode ofi tube Ixll, the; source 31| Vand" the resistor 321 may: be left noa-ting; Y Thesame istrue of' the shield tit.l The reason for thisis that the plasma. produced inv` the auxiliaryu discharge will be so conductive that'- it willA ofi itsL own accordi assume' a xed potential relationship with respect,` to the anode W'. This will resul'tin satisfactory operation. sinceelectros'tatio acceleration. isf not re'- quiredv to move hot. electrons froml the auxiliary dischargethrough; the.forarninous'1pi-.irtion I1 into thespace between-.the. grid'. |3and the anode- Il". It; isl not. required" inasmuch asy the: high thermal velocities? of the. free'electrons. ot the plasma-will suilice to carryfmany of them through the foraminousportion.|11'.v

Referringl again to the embodimentof Figure 2;

it is noted that the 'type of operation involving the cooperative use of the filament I5 andthe subsidiary anode 2| can be obtainedv in -a tube in y lpwhichno subsidiary anode 2|, as such, is 'pro'- vided". For example, this kind of operation 'can be obtained by causing' a small wall area o'f the main anode I4 which is adjacent to the-filament I5 to perform the function of theV omitted subvd .v sidiary" anode. 2 I. In such a case the filament I5 would be polarized at a negative potential with respect to the anode I4 by the use of an energizing circuit like those, already described, which in.-

clude a limiting resistorA 32. Moreover, the same t type of operation can be obtained, if preferred', by

Q similarly causing a small 'area of the outer-sur#- iace of the control grid I3 to perform the function of the omitted anode 2|.

Itis apparent from theforegoing that the prin- V-i ciple` ofv this'v invention can be ideally applied to a no hot cathode in the' tube at alla In other-words,

decay in a restricted portion of a thyratron cause it to. be slow throughout a cold cathode tube. Acicordingly, any of the free'- electron heatingemeans shown herein may be added to such a tube to hasten` its deionization. In the easev of.' a' cold in performance will nevertheless be obtained be'- anode potential has been removed, i. e., theperiod which', in the above example of av sweep circuit, occurs.: immediately after the start of the-storage condenser discharge. As explained above during .thisv period the decaying plasma eiectivel'yshort circuits the storage vcondenser' and thereby delays the start of the next charging cycle. Obviously the shortening off this interval willv improve the cold" cathode gas tube as a switch apart from any f sistor 32 can be very effective for preventingV the auxiliary discharge from producingA too much plasma while acting as a source offhot electrons,

it is less effective for' :increasing the arc-drop'.

A more effective way ofv increasing the arc-drop isto` operate the subsidiary cathode, I5 or.'2`l) as the case may be, with temperature' limited .7,51`emission. By so doing, bothdesirable-ends will be attained: (1) The auxiliary current will vbe kept low, and with it the generation of plasma; and (2) the arc drop will be verysubstantially increased. The greater arc drop willresult in increased electron acceleration(s), i. e., hotter electrons. In this connection it may be noted that two distinct ways of provided hot electrons have been described herein. One is by thermionic emission and the other is by thermionic emission plus acceleration. In the case of the latter, some ionization is unavoidable inasmuch as the auxiliary space discharge occurs through the gaseous lling of the tube. However, the ionization lper se is a side effect, which, while it serves to increase the magnitude of the auxiliary discharge, does so without increasing the temperatures of the electrons beyond their values due to acceleration over the discharge path. However, as was explained above, the generation of plasma in the auxiliary discharge can be tolerated if it is kept low enough.

What is claimed is:

1. A gas tube comprising: a sealed envelope containing an ionizable medium, a cathode and an anode, and means adjacent to said anode and other than said cathode for heating electrons in the region close to said anode to prevent cooling of said electrons said medium in the absence of a gaseous discharge between the anode andcathode.

2. A gas tube comprising: a sealed envelope containing an ionizable medium and in the order named a cathode, a control electrode and an anode; and means adjacent said anode for heating electrons in the region between said control electrode and anode.

3. A gas switch tube comprising: a sealed envelope containing an ionizable medium and' in the order named a cathode, a control electrode and an anode; and means adjacent said anode for heating the electrons in the portion of'said medium between said control electrode and anode; said means comprising an electrical heating element in heat exchange relationship with-the portion of said medium near the anode. A

4. A gas switch tube comprising: a sealed envelope containing an ionizable medium and in the order named a cathode, a control electrode and an anode; and means adjacent said anode for heatingthe electrons in the portion of said medium between said control electrode and anode; said means comprising a resistive lament between said control electrode and anode.

5. A switch tube as in claim 4 in which said lament is a directly heated thermionic cathode.

6 A gas switch tube comprising: a sealed envelope containing an ionizable medium and in the order named a cathode, a control electrode and an anode; and means adjacentrsaid anode forheating the electrons in the portion'of said medium between said control electrode and anode; said means comprising a subsidiary cathode and a subsidiary anode in cooperative spaced relationship for producing an auxiliary gaseous discharge to provide hot electrons to said portion of the medium while generating plasma of very much lower density than that generated between said cathode and anode during current conduction therebetween.

'7. A switch tube as in claim 6in which said subsidiary cathode and anode are located in a region on the far side of said anode from* said control electrode and a portion of said anode between said region and said control electrodegis destinatarie/parent. l

8. A gas switch tube comprising: a sealed envelope containing an ionizable medium and in the order named a cathode, a control electrode and an anode; and means adjacent said anode for heating the electrons ,in the portion of said medium between said control electrode-and anode; said means comprising a thermionic cathode between said control electrode-and anode.

9. A gas switch tube comprising: a sealed envelopecontaining an ionizablemedium-and in the order -na'med` a. cathode, a control electrode and anA anode; and means adjacentv said anode for heating the electrons in the portion Yof said medium between said control electrode and anode;- said ;contr ol electrode including a metal plate having a slot in alignment with said cathode and a portionof said anode and-said'means comprising meansfor providing hot electrons to said port-loniof said medium in a region -thereof near to said-slot. v 1 e 10. A ga's switch tube compri-sing: a Isealed envelope containing an ionizable-medium and in the order named av cathode, a control electrode and an anode; and means adjacent said anode for heating: the,` electrons in the vportion of said medium between said control electrode and anode; said means comprising a subsidiary temperature limiting cathode and a'subsidiary anode in Acooperativespaced relationship for producing an auxiliary space discharge.

1,1-, A gas :switch tube comprising: a sealed envelope containing -an ionizable medium and in the order named 'a cathode, a control electrode and an anode; and means adjacent said anode for heating the electrons in the portion of said medium between said control electrode and anode; said means comprising a subsidiary temperature limited cathode inA cooperative spaced relationship with another-electrode within the envelope toproduce an auxiliary space discharge.

l- ,12.- A- gas switch vtube comprising a sealed envelope-containingv an ionizable medium, al cold cathode, an anode and means for heating the electrons in medium in the absence of a gaseous discharge betweenv the anode and-cathode.

l13. -.A switch tube -as in claimv 12 in which said means comprises a resistive filament.v

lflrgA switch tube-as `in claim 1,3 in which said lament-4 is ya directly heated thermionic cathode.

' 15. A switch tube as in claim 12 in which said means comprises asubsidiary cathode and a subsidiary anode in cooperative -spaced relationship for producing an auxiliary discharge to provide hot electrons to said medium without generating suicient-plasma to support an anode to cathode current-rofgaseous -discharge proportions.

16. A switchtube as in claim 15 which further comprises-a control electrode between said cathode `and anode and inwhich -saidzmeans is located adjacent the anode for heatingaportion oi'- said medium -between said control'electrodeand anode.

-17.- A lswitch tubeasjin-claim 12 in which said means. comprises a subsidiary temperature limited cathode and a subsidiary-anode in cooperative-spaced relationship f or producing an auxiliaryspaced discharge.

18. Gas tube switching-apparatus comprising a sealed envelope containing an ionizable medium, acathode andan anode, and means. other thanA said cathode -for heating 'electrons in said med1um inthe absence of a spacious discharge between said anode and cathode; said means comprising a subsidiary cathode for producing an auxiliary discharge to another electrode within the envelope and an auxiliary discharge energizing circuit connected between the subsidiary cathode and said last-mentioned electrode and ,REFERENCES CITED including a resstoi` for so limiting the current of The f01 10W1I1g references are 0f record 1n the the auxiliary discharge that it cannot generate file 0f thlS patenti suicient plasma to support the anode to cath- 5 UNITED STATES PATENTS ode current carried by the tube when 1t 1s to be Number Name Date actuated as an on-switch. 2 158 564 LOUIS MALTER. Meler May 16, 1939 2,213,551 Nelson Sept. 3, 1940 EDWARD O. JOHNSON. 

